Color and brightness of a painting color applied to a body of a motor vehicle depend on a viewing direction. Such appearance of the painting color can be expressed numerically with the reflection characteristics of a painted plane, namely the reflectance thereof. Therefore, by measuring the reflectance of the painting color, the quality thereof can be adjusted in the producing step and the testing step of the painting color, and the painting color can be examined using computer graphics (CG) prior to painting. Consequently, it is important to measure the reflectance of the painting color. And, it is an important problem how the measuring conditions of the reflectance of the painted plane are set.
It has been known that the reflection characteristics of a general painting color applied to a vehicle body can be sufficiently obtained by measuring the reflectance when light made incident at an incident angle of 60° is received at representative five displacement angles of about 10° (10°±several degree), about 18° (18°±several degree), about 28° (28°±several degree), about 40° (40°±several degree) and about 90° (90°±several degree) (see Patent Document 1, for example).
In this Patent Document 1, when the light made incident on an incident point P on a plane under measurement at an incident angle θ regularly reflects inside an incident plane A at a reflection angle θ, the regularly reflecting direction is expressed by a displacement angle of 0°, and the angle from the regularly reflecting direction toward a normal line at the incident point P on the pane under measurement (the side from the regularly reflecting direction toward the normal line is positive, whereas the side toward the plane under measurement opposite to the normal line is negative) is expressed by a displacement angle.
In this five angles method, reflectances of the light received at representative five displacement angles within a single incident plane are measured and interpolated. With this method, the reflectance of a complicated painting color such as a metallic color, pearl mica, etc. other than a solid color can be measured conveniently.    Patent Document 1: Patent application laid-open No. 2003-307456